Download Metamorphism usually involves changes in

Metamorphism
• Changes in rocks due to increasing P-T conditions and/or
interaction with fluids.
Importance
1. Mineral Resources
2. Mountain Building Events
3. History of Continental Crust
Oldest
Earth
Uncut rocks
Ruby on
andthe
Sapphire
(4.0 billion year old gneiss
from Northern Canada)
Metamorphism usually involves changes in:
• mineralogy  formation of new metamorphic minerals
• texture  development of metamorphic “fabrics”
Mineralogical Changes
Textural Changes
Metamorphic Conditions
• All changes occur in the SOLID state between ~100C and 800 C
“Solid State Recrystallization” = Metamorphism
• Metamorphic “Grade” refers to general P-T conditions
• High-temperature limit grades into partial melting 
migmatites (“mixed rocks”)
Agents of Metamorphism
• Temperature:
depends on geothermal gradient (avg. 30°C/km)
•
Pressure:
1. lithostatic - uniform P, due to weight of overlying
rock; 1 kb (0.1 GPa) = 3.3 km depth.
2. differential - unequal P in different directions;
produces metamorphic rock fabrics
•
Fluids:
H2O-dominated ± CO2. Derived from metamorphic
reactions (internal) or magmatic fluids (external).
Types of Metamorphism
Two main types at tectonically active regions:
(1) Contact Metamorphism
(2) Regional Metamorphism
Contact Metamorphism
• thermal metamorphism due
to heat of igneous intrusions
• narrow zones (<1 km wide)
Regional Metamorphism
• Large, regional areas of crust affected (thousands of km2); one or
more episodes of orogeny with combined elevated geothermal
gradients and deformation
• Associated with mountain building processes at convergent plate
boundaries (subduction zones; collision zones)
Examples: Andes, Himalayas, Appalachians
• Full range of P-T metamorphic conditions; foliated rocks are a
characteristic product
Variable P-T Conditions in a Convergent Plate Setting
Low P, high T
(contact)
high P, low T
(“blueschist”)
high P and T
(regional)
Non-foliated
Foliated
Common Metamorphic Fabrics
Slaty Cleavage
Schistocity
Gneissic Banding
Origin of
Metamorphic
Foliation
Produced by differential stress
Compressive
Shearing
Origin of Metamorphic Foliation
Rotation and flattening
of platy (clays, micas) or
elongate minerals
(hornblende, feldspars)
Granite
Granitic Gneiss
Broad Compositional Categories
based on mineralogy and textures ultimately
inherited from the “protolith”.
“Protolith” = parent rock type prior to metamorphism
Quartz Sandstone
(a) Limestone (fiossiliferous)
Shale
Schist
IMPORTANT CONCEPT:
Metamorphic assemblages are a function of
P-T and protolith chemistry
 Different protoliths will yield different
mineral assemblages at the same P-T
conditions
3 Most Important Compositional Categories
1. Pelites: protolith = Al-rich, fine-grained clastic
sediments (shales, siltstones). Classic slate-phylliteschist-gneiss sequence.
2. Calcareous: protolith = carbonate rocks (limestones,
dolostones, shaly ls). Marbles, calc-silicate rocks.
3. Mafic and Ultramafic: protolith = ultramafic to mafic
igneous rocks. Greenstones, amphibolites, granulites.
P-T Classification
metamorphic grade (low, intermediate, high) is the most
basic way to classify based on P-T
BUT, we can be more
specific than that!
P-T diagram showing
“Metamorphic Facies”
Metamorphic Facies are broad characterizations of the P-T conditions
experienced by metamorphic rocks in an area. They are represented by
“fields” or “polygons” on a P-T diagram.
Adirondacks, NY
NJ Highlands rocks
If we find rocks in the field with a particular mineralogy, then a certain
facies (P-T conditions) may be assigned to the area.
Protolith = mafic igneous rocks
• Facies are defined by distinctive mineral assemblages
• Facies boundaries are defined by important mineral reactions
and the disappearance/appearance of distinctive minerals.